In the copending patent applications cited above respective transmitters and receivers for differentially coherent PSK data transmission systems are disclosed. The transmitter described by me employs a substantially digital implementation of a PSK modulator using a quantized "staircasing" arrangement compatible with large-scale integrated-circuit (LSI) techniques. The output wave produced after low-pass filtering is characterized by a substantially constant frequency with discrete phase changes occurring at synchronous modulation, i.e., baud, intervals. The information to be transmitted is encoded in phase changes measured between the centers of successive modulation, signaling or baud intervals. Due to this differential encoding no absolute phase reference need be transmitted with the output wave.
In his copending application Tong describes a receiver for a differentially coherent PSK signal wave using comparison detection techniques implemented by a multistage shift register and a pair of binary counters. Present and delayed replicas of received signal samples taken in successive baud intervals are correlated in exclusive-OR gates, whose outputs drive the binary counters. According to whether or not the counters have reached predetermined count thresholds the binary nature of the transmitted data bit is ascertained. The instant of time at which the observation of the attainment or not of the count threshold is important from the standpoint of the validity and freedom from error of the resultant data decision. Tong broadly indicates a baud-rate clock for sampling the counter outputs. Tong's digital demodulator is also compatible with LSI techniques.
It is an object of this invention to provide an improved sample timing recovery circuit for differentially coherent PSK data transmission systems.
It is another object of this invention to provide a timing recovery circuit for differentially coherent PSK data transmission systems using digital apparatus compatible with LSI techniques.
It is a further object of this invention to provide a timing recovery circuit for a differentially coherent PSK data transmission system using correlation techniques.